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We present a detailed spectroscopic analysis of the stars withhelium-dominated spectra in the ESO Supernova Ia Progenitor Survey (SPY).Atmospheric parameters, masses, and abundances of trace hydrogen are determinedand discussed in the context of spectral evolution of white dwarfs. Our finalsample contains 71 objects, of which 6 are new detections and 14 arereclassified from DB to DBA because of the presence of H lines. 55% of the DBsample show hydrogen and are thus DBA, a significantly higher fraction thanfound before. The large incidence of DBA, and the derived total hydrogen massesare compatible with the scenario that DBs ``reappear'' around 30000 K from theDB gap by mixing and diluting a thin hydrogen layer of the order of E-15 Msun.This hydrogen mass is then during the evolution continuously increased byinterstellar accretion. There are indications that the accretion rate increasessmoothly with age or decreasing temperature, a trend which continuous evenbelow the current low temperature limit (Dufour 2006). A remaining mystery isthe low accretion rate of H compared to that of Ca observed in the DBZA, but astellar wind extending down to the lowest temperatures with decreasing strengthmight be part of the solution.

High-Resolution Spectra Observed for SPY. II. DB and DBA Stars